This invention relates to slowing motion of objects and, more particularly, to cellular concrete units suitable for use in arresting bed systems to safely decelerate an aircraft which runs off the end of a runway, and methods for producing such units.
Aircraft can and do overrun the ends of runways raising the possibility of injury to passengers and destruction of or severe damage to the aircraft. Such overruns have occurred during aborted take-offs or while landing, with the aircraft traveling at speeds to 80 knots. In order to minimize the hazards of overruns, the Federal Aviation Administration (FAA) generally requires a safety area of 1,000 feet in length beyond the end of the runway. Although this safety area is now an FAA standard, many runways across the country were constructed prior to its adoption and are situated such that water, roadways or other obstacles prevent economical compliance with the one thousand foot overrun requirement.
Several materials, including existing soil surfaces beyond the runway have been assessed for their ability to decelerate aircraft. Soil surfaces are very unpredictable in their arresting capability because their properties are unpredictable. For example, very dry clay can be hard and nearly impenetrable, but wet clay can cause aircraft to mire down quickly, cause the landing gear to collapse, and provide a potential for passenger and crew injury as well as greater aircraft damage.
A 1988 report addresses an investigation by the Port Authority of New York and New Jersey on the feasibility of developing a plastic foam arrestor for a runway at JFK International Airport. In the report, it is stated that analyses indicated that such an arrestor design is feasible and could safely stop a 100,000 pound aircraft overrunning the runway at an exit velocity up to 80 knots and an 820,000 pound aircraft overrunning at an exit velocity up to 60 knots. The report states that performance of an appropriate plastic foam arrestor configuration was shown to be potentially "superior to a paved 1,000 foot overrun area, particularly when braking is not effective and reverse thrust is not available." As is well known, effectiveness of braking may be limited under wet or icy surface conditions. (University of Dayton report UDR-TR-88-07, January 1988.)
More recently, an aircraft arresting system has been described in U.S. Pat. No. 5,193,764 to Larrett et al. In accordance with the disclosure of that patent, an aircraft arresting area is formed by adhering a plurality of stacked thin layers of rigid, friable, fire resistant phenolic foam to each other, with the lower-most layer of foam being adhered to a support surface. The stacked layers are designed so that the compressive resistance of the combined layers of rigid plastic foam is less than the force exerted by the landing gear of any aircraft of the type intended to be arrested when moving into the arresting area from a runway so that the foam is crushed when contacted by the aircraft. The preferred material is phenolic foam used with a compatible adhesive, such as a latex adhesive.
Tests of phenolic foam based arrestor systems indicate that while such systems can function to bring aircraft to a stop, the use of the foam material has disadvantages. Major among the disadvantages is the fact that foam, depending upon its properties, can typically exhibit a rebound property. Thus, it was noted in phenolic foam arresting bed testing that some forward thrust was delivered to the wheels of the aircraft as it moved through the foamed material as a result of the rebound of the foam material itself.
Foamed or cellular concrete as a material for use in arresting bed systems has been suggested and undergone limited field testing in the prior art. Such testing has indicated that cellular concrete has good potential for use in arresting bed systems, based on providing many of the same advantages as phenolic foam while avoiding some of phenolic foam's disadvantages. However, the requirements for an accurately controlled crushing strength and material uniformity throughout the arresting bed are critical and, so far as is known, the production of cellular concrete of appropriate characteristics and uniformity has not previously been achieved or described. Production of structural concrete for building purposes is an old art involving relatively simple process steps. Production of cellular concrete, while generally involving simple ingredients, is complicated by the nature and effect of aeration, mixing and hydration aspects, which must be closely specified and accurately controlled if a uniform end product, which is neither too weak nor too strong, is to be provided for present purposes. Discontinuities, including areas of weaker and stronger cellular concrete, may actually cause damage to the vehicle that is being decelerated if, for example, deceleration forces exceed wheel support structure strength. Such nonuniformity also results in an inability to accurately predict deceleration performance and total stopping distance. In one recent feasibility test utilizing commercial grade cellular concrete, an aircraft instrumented for recording of test data taxied through a bed section and load data was acquired. Even though steps had been taken to try to provide production uniformity, samples taken and aircraft load data from the test arresting bed showed significant variations between areas where the crush strength was excessively high and areas where it was excessively low. Obviously, the potential benefit of an arresting system is compromised, if the aircraft is exposed to forces that could damage or collapse the main landing gear.
A 1995 report prepared for the Federal Aviation Administration entitled "Preliminary Soft Ground Arrestor Design for JFK International Airport" describes a proposed aircraft arrestor. This report discusses the potential for use of either phenolic foam or cellular concrete. As to phenolic foam, reference is made to the disadvantage of a "rebound" characteristic resulting in return of some energy following compression. As to cellular concrete, termed "foamcrete", it is noted that "a constant density (strength parameter) of foamcrete is difficult to maintain" in production. It is indicated that foamcrete appears to be a good candidate for arrestor construction, if it can be produced in large quantities with constant density and compressive strengths. Flat plate testing is illustrated and uniform compressive strength values of 60 and 80 psi over a five to eighty percent deformation range are described as objectives based on the level of information then available in the art. The report thus indicates the unavailability of both existing materials having acceptable characteristics and methods for production of such material, and suggests on a somewhat hypothetical basis possible characteristics and testing of such materials should they become available.
Thus, while arresting bed systems have been considered and some actual testing of various materials therefor has been explored, practical production and implementation of either an arresting bed system which within specified distances will safely stop aircraft of known size and weight moving at a projected rate of speed off of a runway, or of materials suitable for use therein, have not been achieved. The amount of material, and the geometry in which it is formed to provide an effective arresting bed for vehicles of a predetermined size, weight, and speed, is directly dependent upon the physical properties of the material and, in particular, the amount of drag which will be applied to the vehicle as it moves through the bed crushing or otherwise deforming the material. Computer programming models or other techniques may be employed to develop drag or deceleration objectives for arresting beds, based upon the calculated forces and energy absorption for aircraft of particular size and weight, in view of corresponding landing gear strength specifications for such aircraft. However, the models must assume that the arresting bed is constructed of a material having a section to section and batch to batch uniformity of characteristics, such as strength, durability, etc., to produce uniform results with a predictable amount of energy absorption (drag) when contacted by the portions of the aircraft (or other vehicle) which are bearing the load of the vehicle through the bed (e.g., the wheels of an aircraft as it moves through the bed after having overrun the runway).
One of the potential benefits of the use of foamed or cellular concrete in arresting bed systems is that the material itself is capable of being produced in a variety of different ways using numerous different starting materials. For prior types of applications not related to vehicle deceleration the concrete has been produced by using a particular type of cement (usually Portland) which is combined with water, a foaming agent, and air to produce a cellular concrete. However, a significant distinguishing requirement separates such prior applications of cellular concrete from production of a product suitable for use in an arresting bed. In prior applications, the objectives are typically reduced weight or cost, or both, while providing a predetermined minimum strength with the more strength the better. Prior applications have typically not required that cellular concrete be produced to strict standards of both maximum strength and minimum strength. Also, prior applications have not required a high degree of uniformity of material, provided basic strength objectives are met. Even for prior applications of cellular concrete, it is known that the amount and type of cement, the water/cement ratio, the amount and type of foaming agent, the manner in which the materials are combined, processing conditions and curing conditions can all have critical effects on the resulting properties of the cellular concrete. No necessity to refine production to the levels required to produce cellular concrete suitable for vehicle arresting beds has been presented by prior applications.
Thus, it is one thing to specify objectives as to the mechanical properties of materials appropriate to obtain the desired deceleration on entry of an airplane or other vehicle into the arresting bed. However, the capability of consistently producing cellular concrete material which will actually have the required properties of predetermined strength and uniformity is not known to have been previously achieved.
One substantial problem in the art is the lack of established techniques for production of cellular concrete in the low strength range, in a uniform fashion to very tight tolerances, to enable construction of an entire arresting bed consistently having the desired mechanical properties throughout its geometry.
Objects of the invention are to provide new and improved vehicle arresting units and methods for their production which provide one or more of the following characteristics and capabilities:
units produced in block form of sizes suitable for a variety of applications; PA1 units produced to provide predetermined compressive gradient strength characteristics; PA1 units having uniformity of characteristics suitable for safely arresting vehicle travel; PA1 methods enabling repeatable production with predetermined characteristics; PA1 methods enabling production control based on established parameter ranges; and PA1 methods enabling a high level of quality control in production of cellular concrete having predetermined compressive gradient strength suitable for a variety of applications.